Well the internet isn't co-operating and I can't get the OSU website to open up but:
1. Johnson grass does indeed produce cyanide at certain times of the year. My eye doctor lost a cow when the dumb head jumped the fence and got into a big patch of it. He was able to afford an autopsy just to be sure of that.
2. Lots of folks feed horses Johnson grass hay without issue. Evidently ruminant livestock (like cows) are more susceptible to the cyanide (what ever time of year it's produced) than horses.
My horses won't touch a blade of Johnson grass if there happens to be some mixed in their hay.
I have a huge patch of it in corner up by the road. That whole section is fenced off from the horses because:
A) I don't want my horses by the road.
B) I don't want them eating "live" Johnson Grass just in case, because it would be my luck----------------lollol
Actually, two years ago my neighbor baled that 3 acres where the Johnson grass is and fed it to his horses without any problem. Mine wouldn't touch it with a 20 foot pole. He said my horses aren't hungry enough. I said that's ok, just let me know if any of yours cash in over the winter
I said all of that to say, Johnson Grass is not supposed to be dangerous for a horse but I'm not taking the chance; what everyone else wants to do is up to them